3D Bluray support. It's something me and many other Day One owners have been asking for since the start! I can't see how it can be touted as an all-in-one entertainment system if it means I have to go out and buy a third-party Bluray player for my 3D blurays.
The UI is too clunky and confusing. They've been so busy to push the Windows 8 UI on as many devices as possible that it's ruined the Xbox One - possibly irretrievably so. It's well documented that the UI saps CPU power (kinect slices and "snap" capabilities etc.), it launched without some of the basic functionality we saw on the 360 (friend notifications, controller battery indicator) and even 6 months after launch is still being fixed.
Performance parity with PS4. This has to be a priority for Microsoft. That has become the biggest talking point regarding next-gen gaming. Gaming websites no longer talk about the playability or unique selling point that games offer. Headlines for games now tell you the title of the game and what resolution they run on PS4 compared to Xbox One. If it ain't in the headline, it literally 2 or 3 sentences into the review or article. Microsoft need to ditch whatever they need to to see that parity is achieved, and clearly ditching Kinect is a step in the right direction.
I like my Xbox One, but it could have been a lot better. Microsoft have dropped the ball and it may be too late to retrieve it...